Flake wrote in a tweet that he is "working with [Feinstein] on a bipartisan bill that will raise the minimum purchase age for non-military buyers from 18 to 21 — the same age you currently have to be to purchase a handgun."

The GOP senator tweeted that a "kid too young [to] buy a handgun should be too young to buy an #AR15."

A kid too young buy a handgun should be too young to buy an #AR15. Working with @SenFeinstein on a bipartisan bill that will raise the minimum purchase age for non-military buyers from 18 to 21 - the same age you currently have to be to purchase a handgun.

Feinstein announced last week, following a mass shooting at a Florida high school, that she would introduce legislation raising the minimum age for buying rifles, effectively requiring anyone buying a firearm from a dealer to be at least 21 years old.

News of bipartisan support for such legislation comes after 17 people were killed and more than a dozen others injured during a shooting at Marjory Stoneman Douglas High School in Parkland, Fla., roughly 30 miles from Fort Lauderdale.

The suspected shooter, 19-year-old Nikolas Cruz, is said to have used an AR-15. Federal law enforcement officials told The New York Times that the gun was purchased legally.

Under federal law, gun dealers can't sell a handgun to anyone under the age of 21.

But licensed gun dealers can sell "long guns," which include rifles, to anyone 18 and older under federal law, with most states having similar age restrictions, according to the Giffords Law Center to Prevent Gun Violence.

Despite support from Flake, who announced he would retire from the Senate after this year, Feinstein's forthcoming bill could face an uphill fight in a GOP-controlled Congress where gun legislation has stalled for years.

Trump has also asked the Department of Justice to propose regulations that would ban bump stocks, a device that can simulate automatic gunfire with a semi-automatic weapon that drew attention following a deadly shooting in Las Vegas last year.